This invention relates to a sensor for sensing acceleration and a sensing system for sensing acceleration.
U.S. Pat. No. 4,843,877 discloses an acceleration sensor which comprises a tubular bobbin, a steel ball (magnetic member) received within the bobbin, a differential transformer (detection means) mounted around the bobbin, and a permanent magnet disposed radially outwardly of the bobbin. The acceleration sensor further comprises a housing, and the bobbin with the differential transformer and the permanent magnet are immovably mounted within the housing, together with a spacer.
In the above conventional acceleration sensor, a reference position of the steel ball (that is, the position of the steel ball when the sensor is subjected to no acceleration) is determined by the permanent magnet. When the sensor is subjected to acceleration, the steel ball is displaced to a position where an inertia force tending to move the steel ball from the reference position is balanced with the force of the permanent magnet tending to bring back the steel ball to the reference position. The displacement of the steel ball from the reference position is detected by the differential transformer, thereby sensing the acceleration.
It is desirable that a differential output of the above differential transformer is zero when acceleration is zero. Therefore, the relative position between the differential transformer and the permanent magnet within the housing is so determined that the position of the steel ball where the differential output of the differential transformer is zero (that is, a magnetic center of the differential transformer in the direction of the axis of the bobbin) coincides with the reference position determined by the permanent magnet.
In the acceleration sensor of the above construction, temperature variations have not been sufficiently taken into consideration. More specifically, since thermal expansion coefficients of the housing, the bobbin and the spacer are different from one another, the relative position between the differential transformer and the permanent magnet is varied, though slightly, so that the magnetic center of the differential transformer is brought out of registry with the reference position. This will now be described with reference to the case where the thermal expansion coefficients of the bobbin and the spacer are higher than that of the housing. In the above acceleration sensor, the housing, the bobbin and the spacer are not fixed relative to one another, and when the temperature drops, a small gap develops between any two of these three parts, thus allowing a small amount of movement of the bobbin (to which the differential transformer attached), as well as a small amount of movement of the permanent magnet. Also, when the temperature rises, the housing limits a thermal expansion of the bobbin, so that the magnetic center of the differential transformer is displaced with respect to the reference position along the direction of the axis of the bobbin.
In addition, since the direction and amount of this displacement are not constant, it is difficult to effect a temperature compensation. Further, in the above acceleration sensor, thermal stresses repeatedly develop between the bobbin and the housing, so that the life time of the bobbin is shortened.
Temperature compensations in different types of acceleration sensors are disclosed in Japanese Laid-Open Patent Application No. 213280/87 and Japanese Laid-Open Utility Model Application Nos. 181917/88, 188563/88 and 42460/89.
With respect to other types of accleration sensors different in type from that of the present invention, Japanese Laid-Open Utility Model Application No. 49740/87 discloses the type of acceleration sensor in which a liquid is filled in a sensor housing, and Japanese Laid-Open Utility Model Application No. 187058/88 discloses the type of acceleration sensor in which a circuit board within a housing is fixed by a potting material.